Atomic layer deposition (ALD) processes are used to produce thin, conformal films having high thickness accuracy and step coverage. ALD utilizes a series of sequential, self-limiting surface reactions, each forming a monolayer (e.g., a layer one atom thick) of material, to form the film. ALD provides atomic layer control and enables the films to be successfully deposited on structures having high aspect ratios. ALD conventionally uses two or more gaseous precursors, each being sequentially introduced into a reaction chamber. A wide variety of materials may be deposited by ALD, many of which are used in the fabrication of semiconductor devices. Silicon dioxide (SiO2), a commonly used electrically insulative material in semiconductor device fabrication, may be readily deposited by ALD. As used herein, the term “ALD SiO2” means and includes silicon dioxide formed by an ALD process.
Silicon dioxide formation by ALD is conducted at temperatures from about 320° C. to about 530° C. by sequentially exposing a substrate to a gaseous, silicon-containing precursor, such as tetrachlorsilane (SiCl4), and an oxygen-containing precursor, such as water. One possible mechanism for the surface reactions of the tetrachlorsilane (SiCl4) and water is as follows:SiOH*+SiOx→SiOSiCl3*+HCl  (Reaction 1)SiCl*+H2O→SiOH*+HCl  (Reaction 2)where * indicates a surface species.
Forming silicon dioxide by ALD includes exposing the substrate, which is located in a reaction chamber, to the silicon-containing precursor to accomplish chemisorption of silicon species onto the substrate. Theoretically, the chemisorption forms a silicon-containing monolayer that is uniformly one atom or molecule thick on the entire, exposed substrate. Excess silicon-containing precursor is purged from the reaction chamber and the substrate is exposed to the oxygen-containing precursor. The oxygen-containing precursor chemisorbs onto the silicon-containing monolayer, forming an oxygen-containing monolayer. Excess oxygen-containing precursor is then purged from the reaction chamber. These acts are repeated to form silicon dioxide having a desired thickness of the substrate. The silicon- and oxygen-containing precursors may be mixed with a catalyst, such as pyridine, to speed up deposition while decreasing the reaction temperature in a range of from about 50° C. to about 100° C. Depositing silicon dioxide films at low temperatures is advantageous in several circumstances due to the thermally sensitive nature of substrates or materials deposited thereon.
Purging of the reaction chamber may involve a variety of techniques including, but not limited to, contacting the substrate, monolayer, or silicon dioxide with an inert gas and reducing the pressure within the reaction chamber such that the concentration of reactants (silicon-containing precursor, oxygen-containing precursor, catalyst) in the reaction chamber is reduced. Examples of inert gases include, for example, nitrogen (N2), argon (Ar), helium (He), neon (Ne), krypton (Kr), and xenon (Xe). In conventional ALD processes, over half of the processing time needed to deposit silicon dioxide by ALD may be spent in pump/purge cycles between reactions to minimize parasitic chemical vapor deposition (CVD) reactions. Thus, the processing time may be reduced by decreasing the pump/purge time. However, decreasing the pump/purge time is problematic due to the incomplete removal of reactants and reaction by-products which may lead to: i) a non-uniform thickness of the film across the substrate; and ii) the formation of defects on the surface of the silicon dioxide formed by ALD. The defect formation is especially enhanced when silicon dioxide is deposited on structures including narrow features or openings, such as during silicon dioxide spacer formation.
As an example, a cycle of an ALD process for forming silicon dioxide may include pulsing the silicon-containing precursor material, such as hexachlorodisilane (HCD), or other suitable silicon-containing precursor, into the reaction chamber for about 10 seconds, pumping the reaction chamber for about 10 seconds, purging the reaction chamber by pumping an inert gas into the reaction chamber for about 10 seconds, repeating the pump/purge sequence, pulsing an oxygen-containing precursor, such as water, into the reaction chamber for about 20 seconds, pumping the reaction chamber for about 10 seconds, pulsing the inert gas into the reaction chamber for about 10 seconds, and again repeating the pump/purge sequence. The pyridine catalyst may be flowing during either half reaction or both. This cycle may be repeated until the desired thickness of the SiO2 is achieved in a single layer or other configuration. The total cycle time is about 118 seconds, about 80 seconds of which is pump/purge time.
To reduce fabrication time and cost, it may be desirable to reduce the pump and purge times during the ALD SiO2 process. As an example, the modified sequence may include introducing a pulse of the silicon-containing precursor material, such as HCD or other suitable silicon-containing precursor, into the reaction chamber for about 10 seconds, pumping the reaction chamber for about 3 seconds, pulsing an inert gas into the reaction chamber for about 3 seconds, pulsing the oxygen-containing precursor, such as water, into the reaction chamber for about 20 seconds, pumping the reaction chamber for about 3 seconds and pulsing nitrogen gas into the reaction chamber for about 9 seconds. Thus, the total time for conducting a single ALD cycle is about 56 seconds, about 18 seconds of which is pump/purge time. Therefore, reducing pump/purge time may substantially reduce processing times associated with ALD SiO2 processes.
However, reducing pump/purge times during ALD SiO2 processes causes an undesirable increase in defect formation on the surface of silicon dioxide, especially in narrow regions between the silicon dioxide film. Increased defect density and probability has been observed in devices with a 35 nm feature size as compared to devices with a 50 nm feature size. While not wishing to be bound by any particular theory, it is believed that the defects may be formed as a result of chloride- (Cl), fluoride- (Fl), or ammonium- (NH4+) containing by-products trapped in the narrow regions.
ALD processes may be used to form silicon dioxide on a substrate with precise thickness control and uniformity. FIG. 1 shows a semiconductor structure 100 including a substrate 102 having a plurality of discrete structures 106 with narrow regions 108 therebetween. ALD SiO2 104 is formed over and in contact with the substrate 102. After formation of ALD SiO2104, the narrow regions 108 may have a width “W” in a range of from about 10 nm to about 100 nm. As ALD cycle times continue to decrease, the narrow regions 108 between the plurality of discrete structures 106 become ever more significant as potential nucleation sites for the formation of defects. In other words, as the width W of the narrow regions 108 decreases, the defect density and probability increase. An increased defect density has been observed for narrow regions 108 having a width W of 35 nm compared to narrow regions 108 having a width W of 50 nm. Such defects may adversely affect semiconductor fabrication processes, for example, by interfering with subsequent etching or patterning processes.
Accordingly, there is a need in the art for improved methods of forming silicon dioxide by ALD so that silicon dioxide structures can be scaled to smaller feature sizes without leading to higher defectivity.